The sensory transduction system of Phycomyces blakesleeanus has been studied for some time with the major emphasis placed on characterizing its behavior upon light stimulation. Mutants with modified behavior have become available. Another response is avoidance: when an object is placed near the growing zone of a sporangiophore, it bends away. We have previously shown this to be mediated by a gas which is emitted by the sporangiophore and whose concentration profile is modified by the presence of objects or by winds; the resulting concentration asymmetry is detected in the growing zone and induces the bending response. A very likely candidate for the effector gas has been identified. Labelled effector will be synthesized and used to isolate and characterize receptor protein. The second part of the proposal is the further elucidation of the role of cyclic AMP in sensory transduction and development. Dark levels of cyclic AMP in Phycomyces sporangiophore are very high, decrease precipitously upon light stimulation and recover after ten min. We shall examine the in vivo levels of cyclic AMP of mutants and wild type during certain programs of light and avoidance stimuli. Also studies of adenyl cyclase, and to a lesser extent phosphodiesterase and protein kinase, shall be continued. Adenyl cyclase may provide a "handle" for isolating the elusive blue sensitive photoreceptor.